"London" was often stamped on shotgun barrels for marketing purposes. That is not necessarily an assurance of quality.
It may be Damascus or just treated to look like it.
It may or may not be safe to shoot with black powder loads.
Before he tries shooting it should be checked out by a gunsmith that specializes in muzzle loading guns. If he doesn't know one, contact me off forum, I may be able to put him/you in touch with one.
A picture would be nice.